Selective catalytic reduction (SCR) provides a method for removing nitrogen oxides (NOx) emissions from fossil fuel powered systems for engines, factories, and power plants. During SCR, a catalyst facilitates a reaction between exhaust-gas ammonia and NOx to produce water and nitrogen gas, thereby removing NOx from the exhaust gas.
The ammonia that is used for the SCR system may be produced during the operation of the NOx-producing system or may be stored for injection when needed. Because of the high reactivity of ammonia, storage of ammonia can be hazardous. Further, on-board production of ammonia can be costly and may require specialized equipment.
One system for on-board ammonia production is disclosed in U.S. Pat. No. 5,964,088, issued to Kinugasa on Oct. 12, 1999 (hereinafter the '088 patent). The system of the '088 patent includes an engine having first and second cylinder groups being connected to first and second exhaust passages. The first exhaust passage includes an ammonia (NH3) synthesizing catalyst, and the two exhaust passages are merged downstream at an interconnecting exhaust passage, which includes an exhaust purifying catalyst. An additional engine, which performs a rich operation, is provided, and the exhaust gas from the additional engine is fed to the first exhaust passage upstream of the ammonia synthesizing catalyst to make the exhaust gas air-fuel ratio of the exhaust gas flowing into the ammonia synthesizing catalyst rich to thereby synthesize ammonia.
While the method of the '088 patent may reduce NOx from an exhaust stream through use of on-board ammonia production, the method of the '088 patent has several drawbacks. For example, the method of the '088 patent may produce high-temperature exhaust gas and catalyst exotherms, which may heat the ammonia synthesizing catalyst. Excessive heating of the ammonia synthesizing catalyst may cause the ammonia synthesizing catalyst to function less efficiently. Further, the '088 patent does not provide a method for controlling the temperature of the ammonia synthesizing catalyst to provide efficient ammonia production.
The present disclosure is directed at overcoming one or more of the shortcomings of the prior art ammonia production systems.